does more processors and cores help performance?
When it come to rendering, the key is the number of cores : two core are twice faster than one core. The frequency of each core is to be considered too.

The bad news is processors are expensive. Sure, a 1,000$ i9 will render faster than your i3 ; but not that fast. And motherboards with 2 sockets aren't cheap either.

The good news is, graphic cards have a lot of cores and are relatively cheap (thanks to the gaming industry). Each core is less powerful than a CPU one, but there are literally hundreds of cores. And nearly every motherboard allows to use two or more graphic cards. There are chances that you already own a compatible graphic card.

A good way to know which cards are best is to compare them using this benchmark : http://blenchmark.com/gpu-benchmarks

To use GPU instead of CPU for rendering, you need to set it in the User Preferences. In System/Cycles Compute Device, choose CUDA (for a NVidia card) or OpenCL (for AMD cards). This question has a really detailed answer about that topic.